Make sure we always extract date/time in UTC

Summary:
Now it matters even for invalid cases, so those one slipped through
before. I admit it makes me wonder why I keep going for the pain of
date/time while we really only allow the user to change the date only.
:-)

One more test suite passing.

Reviewers: bensi, franckarrecot, dfaure, mlaurent

Differential Revision: https://phabricator.kde.org/D876